JPH08222223A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE: To obtain a nonaqueous electrolyte secondary battery having a long service life. CONSTITUTION: LiMx Co1-x O2 (in which M shows at least one sort of transition metal other than Co, and 0<=x<=1.0) is used as an active material, and a positive electrode including a conductive agent and binder, together with the active material, a negative electrode, a separator provided between both electrodes, and a nonaqueous electrolyte liquid held in the separator are provided. And the specific surface area of the positive electrode active material is made 0.35 to 2.0m<2> /g.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte battery such as a lithium secondary battery.

[0002]

2. Description of the Related Art In recent years, electronic devices such as portable wireless telephones, portable personal computers and portable video cameras have been developed, and various electronic devices have been miniaturized to a portable size. Along with this, as a built-in battery, a battery having a high energy density and being lightweight is adopted. A typical battery satisfying such a requirement is a lithium-based material such as lithium metal, a lithium alloy, or carbon having lithium ions retained as a negative electrode active material, and an aprotic material in which a lithium salt such as LiClO ₄ or LiPF ₆ is dissolved. Is a lithium secondary battery using the organic solvent as an electrolyte.

A lithium secondary battery is a negative electrode plate formed by holding the negative electrode active material on a negative electrode current collector which is a support thereof, and a lithium cobalt composite oxide causes a reversible electrochemical reaction with lithium ions. A positive electrode plate that holds the positive electrode active material on a positive electrode current collector that is a support thereof, and a separator that holds an electrolytic solution and that is interposed between the negative electrode plate and the positive electrode plate to prevent a short circuit between both electrodes. There is.

By the way, there has been proposed a technique for specifying the average particle size of the positive electrode active material in order to increase the charge / discharge voltage and energy density of the lithium secondary battery (Japanese Patent Laid-Open No. 1-33)
04664, JP-A-4-162357).

[0005]

However, as a result of research conducted by the inventor of the present application, it was found that there is a large difference in battery performance depending on the particle size distribution even if the average particle size is the same. Therefore, an object of the present invention is to specify a positive electrode active material from a viewpoint different from the prior art for specifying the average particle size, and to provide a non-aqueous electrolyte secondary battery having excellent battery performance.

[0006]

In order to achieve the object, the non-aqueous electrolyte secondary battery of the present invention comprises LiM _x Co _1-x.
A positive electrode containing O ₂ (M represents at least one kind of transition metal other than Co, and 0 ≦ x ≦ 1.0) as an active material, and a conductive agent and a binder in addition to the active material, In a secondary battery including a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolytic solution held by the separator, the positive electrode active material has a specific surface area of 0.35 to 2.0 m ² / g. Is characterized by.

Here, specific examples of LiM _x Co _1-x O ₂ include LiCoO ₂ , LiMn _0.15 Co _0.85 O ₂ , and LiN.
i _0.10 Co _0.90 O ₂ , LiNi _0.37 Co _0.63 O _{2 and the} like.

In this non-aqueous electrolyte secondary battery, it is preferable that the half widths of the (003) plane peak and the (104) plane peak in the X-ray diffraction pattern of the positive electrode active material are both 0.50 or less. It is particularly preferable that both widths are 0.10 to 0.20.

[0009]

The electrode plate contains a conductive agent for electrically connecting the active materials and a binder for holding the active material on the current collector. Therefore, the content of the active material is relatively reduced by the content of the conductive agent and the binder, and the battery capacity is smaller than that of a theoretical battery of the same volume containing no conductive agent or binder. ing.

In view of the above, in the present invention, attention is paid to the fact that the surface of the active material is covered with the conductive agent and the binder, and the specific surface area of the positive electrode active material is set to 2.0 m ² / g or less. The content of the conductive agent and the binder which come into contact with the surface of the active material is the minimum necessary amount, particularly 8% by weight or less for the conductive agent,
It is possible to secure a relatively large content of active material.

On the other hand, if the specific surface area of the positive electrode active material is too small, the reaction area between the active material and the electrolytic solution becomes small, the current density becomes excessive, and the voltage drops when discharging a large current, The capacity will be low. Therefore, in the present invention, the specific surface area of the positive electrode active material is set to 0.35 m ² / g or more.

That is, according to the present invention, by limiting the specific surface area of the positive electrode active material to the range of 0.35 to 2.0 m ² / g, the required reaction area can be secured and the active material content can be large. In addition, the battery capacity and life have been improved.

In addition, by limiting the half width of the X-ray diffraction peak as described above, the battery capacity and life are further improved. That is, it is known that the full width at half maximum represents the degree of crystallinity, and when the width at half maximum is small, the degree of crystallinity is high, and when the width at half maximum is large, the degree of crystallinity is low. Therefore, by limiting the half-value width to the optimum range, the crystallinity of the active material is made appropriate, Li ions are easily moved while maintaining a high packing density of the active material particles, and as a result, a large capacity is obtained. And, it enabled large current discharge. Therefore, when the full width at half maximum is larger than 0.50, the crystallinity is too low, so that the specific gravity of the active material particles is light, the packing density is low, and the capacity is small. However, when the full width at half maximum approaches 0, the crystallinity is so high that there are no gaps, so Li ions do not move easily, and a large current cannot be discharged, or the voltage drop when discharged is significantly large. Become.

[0014]

【Example】

-Example 1- An example in which the present invention is applied to a prismatic lithium ion secondary battery will be described. 20 μm thick as a support for positive electrode plate
Of aluminum foil was prepared. Then, on a weight basis, lithium cobalt composite oxide LiCoO ₂ as an active material
87%, acetylene black powder 5% as a conductive agent, and polyvinylidene fluoride PVDF 8% as a binder
-Mixed with 44% methylpyrrolidone to make a paste. The specific surface area and the half width of the X-ray diffraction peak of the active material used are shown in Table 1.

[0015]

[Table 1] Next, the paste was applied on both sides of the support to a thickness of about 250 μm and dried. Subsequently, two heat rolls arranged in parallel at an interval of 150 μm were rotated so that the feed speed was 1 m / min, and the support coated with the paste was rolled between the heat rolls. A heater is built in the mandrel of the heat roll, and the roll surface during rolling is 1
The conditions were set in advance so that the temperature became 50 ° C.
After rolling, a laminate of the paste and the support was cut into a width of 30 mm to manufacture three types of positive electrode plates having the same shape and quality except that the specific surface area and crystallinity of the active material were different.

Separately, as a support for the negative electrode plate, a thickness of 20
A copper foil of μm was prepared. Then, on a weight basis, 90% of graphite as an active material and 10% of PVDF as a binder were mixed with 56% of n-methylpyrrolidone to prepare a paste. A negative electrode plate was manufactured by applying the paste on both sides of the support, drying, rolling, and cutting into a width of 31 mm in the same manner as in the method for manufacturing the positive electrode plate.

As the electrolytic solution, 1 mol / LiPF ₆ is used.
l-containing ethylene carbonate: diethyl carbonate:
A mixed solution of dimethyl carbonate = 2: 1: 2 (volume ratio) was used. The separator has a thickness of 25 μm and a width of 3
A 2.5 mm polyethylene microporous membrane was used.

Then, the positive electrode plate, the separator and the negative electrode plate are stacked in this order and wound around the polyethylene core in an elliptical spiral shape, and then electrically connected to the positive electrode lead or the negative electrode lead to form a battery. A secondary battery was manufactured by housing in a case, injecting an electrolytic solution, and then welding and covering a necessary part.

The battery thus obtained was charged with a constant current and constant voltage of 200 mA-4.1 V at 25 ° C. for 3 hours, and then 400
The cycle of constant current discharge up to 2.75 V at 25 ° C. at 25 ° C. was repeated, and the battery capacity was measured every 50 cycles.

FIG. 1 shows a graph plotting the relationship between the battery capacity and the number of charge / discharge cycles. In the figure, the symbols Δ, ● and ○ are the numbers. 1, No. 2 and No. The data which used the positive electrode active material of 3 are shown.

As can be seen from the figure, No. 1 having a smaller specific surface area and a larger half width than the range of the present invention. In the battery using the active material of No. 3, the capacity decreases remarkably as the number of cycles increases. The battery using the active material of No. 2 has a gradual decrease in capacity, further has a large specific surface area and a small half width. It was recognized that the battery using the active material of No. 1 tended to maintain a high capacity.

Example 2 In this example, the positive electrode active material has a specific surface area of 1.4 m ² /
g, lithium-nickel-cobalt composite oxide L having a (003) plane half-width of 0.20 and a (104) plane half-width of 0.20
iNi _0.10 Co _0.90 O ₂ was used. A battery was manufactured under the same conditions as in Example 1 except for the above, and the charge / discharge cycle was repeated. As a result, the discharge capacity at the 300th cycle was 400 mAh, and No. 1 Li
The performance almost equal to that of the battery using CoO ₂ was obtained.

Example 3 In this example, the positive electrode active material has a specific surface area of 1.8 m ² /
g, a lithium cobalt composite oxide LiC having a (003) plane half-width of 0.105 and a (104) plane half-width of 0.120
oO ₂ was used. In addition, the composition of the positive electrode mixture was changed to LiCoO ₂ 8
5%, acetylene black powder 7% and PVDF 8%. Other conditions were the same as in Example 1 to manufacture a battery, and the charge / discharge cycle was repeated. As a result, 300
The discharge capacity of the first cycle was 390 mAh, and the No. 1 of Example 1 was used. Performance almost equal to that of the battery using LiCoO ₂ of No. 1 was obtained.

Comparative Example 1 In this example, the specific surface area of the positive electrode active material is 2.2 m ² /
g, a lithium-nickel-cobalt composite oxide LiNi _0.37 Co _0.63 O ₂ having a half-value width of 0.203 on the (003) plane and a half-value width of 0.215 on the (104) plane was used. Other conditions were the same as in Example 1 to manufacture a battery, and the charge / discharge cycle was repeated. As a result, the discharge capacity in the first cycle was 410 mAh, which was inferior in performance to any of the batteries using LiCoO ₂ of Example 1.

Comparative Example 2 In this example, the specific surface area of the positive electrode active material is 0.28 m ² /
Lithium nickel cobalt composite oxide LiNi _0.37 Co _0.63 O ₂ having a half-value width of 0.097 for the (003) plane and a half-value width of the (104) plane of 0.100 was used. Other conditions were the same as in Example 1 to manufacture a battery, and the charge / discharge cycle was repeated. As a result, the voltage increased only to 2.20V from the discharge at the first cycle.

Comparative Example 3 In this example, the positive electrode active material has a specific surface area of 2.3 m ² /
g, lithium-nickel-cobalt composite oxide L having a half width 0.51 of the (003) plane and a half width 0.52 of the (104) plane
iNi _0.37 Co _0.63 O ₂ was used. The other conditions were the same as in Example 1 except that the positive electrode active material was scattered during the paste preparation and the positive electrode active material was aggregated in the paste, so that the paste was uniformly applied to the support. could not.

[0027]

As described above, the non-aqueous electrolyte secondary battery of the present invention has a long life.

[Brief description of drawings]

FIG. 1 is a graph plotting the relationship between charge / discharge cycles and battery capacity.

Claims (4)

[Claims] 1. LiM _x Co _1-x O ₂ (M represents at least one kind of transition metal other than Co, and 0 ≦ x ≦ 1.0) is used as an active material, and conductivity is provided in addition to the active material. In a secondary battery including a positive electrode containing a binder and a binder, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolytic solution held by the separator, the specific surface area of the positive electrode active material is 0. 35-
A non-aqueous electrolyte secondary battery, which is 2.0 m ² / g. 2. The (0
The non-aqueous electrolyte secondary battery according to claim 1, wherein the half-value widths of the (03) plane peak and the (104) plane peak are both 0.50 or less. 3. (0 in the X-ray diffraction pattern of the positive electrode active material
The non-aqueous electrolyte secondary battery according to claim 1, wherein the half widths of the (03) plane peak and the (104) plane peak are both 0.10 to 0.20. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the conductive agent in the positive electrode is 8% by weight or less based on the whole solid substance constituting the positive electrode.